Circuits for electric discharge devices



March 18, 1924. 1,487,451

J. F. FARRINGTON CIRCUTTS FOR ELECTRIC DISCHARGE D EVICES Filed 0012- 16 1922 2 Sheets-Sheet 1 l vvemfan March 18, 1924.

J. F FARRINGTON CIRCUITS FOR ELECTRIC DISCHARGE DEVICES Filed Oct. 16. 1922 2 Sheets-Sheet E.

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Mvenixf/ Patented ar. ltd, 1924.

JDJHN F. FARRINGTON, OI NE'W 1,483,451 ric.

YORK, N. Y., ASSIGNOR T0 WESTERN ELECTRIC COH- JPANY, INCORPORATED, OF NEW YGRK, I i/Y A CORPORATION 035: NEW YORK.

CIRCUITS FOR ELECTRIC DISCHARGE DEVICEfi.

Application filed @ctober 18, 1922. Serial No. 594,70.

To all whom, it may concern."

Be it known that I, JOHN F. FARRINe'roN, a citizen of the United States, residing at New York, in the county of Bronx, State of New York, have invented certain new and useful Improvements in Circuits for Electric Discharge Devices, of which the following is a full, clear, concise, and exact descri tion.

is invention relates to circuit arrangements for heating cathodes of electric discharge devices by alternating current.

An object of the invention is to provide a circuit which will permit the use of alternating current for heating the cathodes of electric discharge devices as, for example, the usual filamentary electron emitters without introducing an objectionable disturbance in the output circuit of the discharge device.

Another object of the invention is to permit the use of alternating current for heating cathodes of discharge devices having a curved operating characteristic without, at the same time, introducing disturbing electromotive forces of the frequency of the heating current or of its harmonics.

A related object is the provision for balancing out the effects of fluctuations in the discharge current of an electron discharge device, such as occur, for example, when rectified alternating current is supplied to the anode of such a device.

A further object of the invention is to arrange the means provided for preventing disturbing components to be introduced due to the heating current, so that this means also acts on the current to be utilized as by amplifying it.,

In many types of electric discharge devices such as those used for X-ray production, rectifying systems and for translating signals, the cathode may be electrically heated to enable it to readily emit electrons. It is frequently very desirable to be able to use alternating current for this purpose. In the case of the well-known three-element electric discharge device having a cathode and an impedance control element to discharge current as a function of the potential difference existing between the cathode .and control element, if alternating current is used for heating the filament, the potential difference between the control element and any point in the filament will rise and fall with the alternation of the heating current. In order to maintain the common potential difference between the grid and the cathode and also that between the anode and the cathode approximately constant, it has been proposed to connect the control element or grid and the anode effectively to the electrical midpoint of the cathode. Such a midpoint connection, although desirable, is insufficient to remedy the difliculty which arises with electric discharge devices operated with a curved characteristic, that is, in such manner as to produce output components, the frequencies of which are combinations of the frequencies of the impressed currents or harmonics thereof. With snlol-i electric discharge devices, the effectsof the two parts of the filament alternately preponderate and cannot therefore completely neutralize each other. It follows that if alternating current be used for heating the cathodes of such devices as modulators or detectors, undesirable combination frequency currents may be produced as well as disturbances of the frequencies of the alternating heating current and its harmonics.

It has also been suggested that the disturbing effect arising from the use of alternating current for heating the cathode may be neutralized by the use of the so-called push-pull or balanced amplifier which has a linear characteristic, that is, it produces in its output circuit amplified currents which are substantially directly proportional to the impressed input electromotive force. These balanced amplifying arrangements are designed so as to prevent intermodulation of the waves impressed thereon. In balanced circuits of familiar type, if the divided output circuits are so connected as to yield the intermodulation components as in the case of balanced modulators and detectors, the disturbances introduced by the alternating current used for cathode heating will not be neutralized and will seriously impair the quality of the resultant waves.

According to the present. invention, the disturbances which arise in the output circuit of an electric discharge device in consequence of the alternating current used for heating its cathode may be neutralized by opposing thereto like disturbances produced in the output circuit of a similar discharge device.

Also, according to the invention, the disturbances in the output circuit of such a device caused by the use of an unsteady source of anode current, for example, rectified alternating current, may be slmilarl neutralized by opposing thereto like disturbancesin the output circuit of a similar discharge device, ,Fnrther according to the invention, the second andsimilar dischar e device, in addition to neutralizin distur ing components in the output 0 the first device, may serve to' perform a further function such as ampli ing the currents or components to be utilized.

Y The invention is illustratedin connection with a radio system, but it is also applicable to wire transmission systems, andto vacuum tube circuits or eleetronrdischarge devices in general.

In thedrawing Fig. 1 illustrates a sim le detector circuit in which use is made 0 a compensating device; Fig. 2 a modificationin which the compensating device serves also as a high frequency amplifier; Fig. 3 a circuit providing both high frequency and low tector circuit; Fig. 5 a modification of Fig.

4; and Fig. 6 a circuit in which the detector is of the regenerative-type. v

The designations of the'various discharge devices indicate their functions e. 'g. D, detector or demodulator, H. F. A. high frequency amplifier, R, rectifier, C, compensating device, and in cases where the coinpensating device serves also as an amplifier, the symbol C is used.

Referring to Fig. 1, a receiving circuit 1, which may be an antenna, is shown inductively coupled to a similarly tuned closed receiving circuit 2 which is included in the input circuit of the detector tube 3. It is immaterial to this invention whether the carrier waves received at the terminal 1 are radiated waves or are waves received over a transmission line. The detector 3 is preferably a thermionic discharge device having a cathode rendered active to emit electrons by being heated, and having the usual anode and control electrode. A compensating discharge device 8 having similar characteristics to the detector tube 3 is shown associated with the detector. A source 4 of alternating current, which may have any desired frequency but which is here assumed to have an audible or sub-audible frequency, serves to supply energy for heating the filament cathodes of the devices 3 and 8, this source being associated with the filaments through a transformer. A potentiometer resistance '5 is brid ed across the filaments and a tap is provided for associating the grids and the plates of the res ective tubes with the center of emissivity-o the cathodes. If the filaments have uniform emissivity for units of len th and are symmetrically associated with the terminals of the resistance 5 the point which represents the center of emissivity of the filaments will be the center of the resistance. In case either or both filaments do not have uniform emissivity per unit len th, compensating resistances 9 may be included, as shown, in series with the filaments for enabling aip'oint to be found on the resistance 5 which represents the center of emissivity of the filaments. The resistances 9 may be adjusted to keep the total resistance of ea'ch filament circuit a constant notwithstanding variations in resistance between a terminal of the filament and a terminal of the resistance 5. Included between the resistance 5 and the respective-plates of the devices 3 and 8 is a plate battery B which supplies anode current to the device 3- through a portion of the resistance 6 and the primary of transformer 7, and space current to the device. 8 through a portion of the resistance 6 and the primary of the transformer 10. The secondary windings of transformers 7 and 10 are connected to the receiving telephones 12.- The grid circuit of the compensating device 8 may, if desired, be provided with a tuned circuit 11 for balancing purposes, which may-be identical with the input circuit 2 of the detector 3.

The resistance 6 aids in obtaining an accurate balance of the circuit. Condenser 13 is provided to by-passthe radio-frequency.

components in the output of the detector 3, and is balanced by the corresponding condenser 14 associated with the tube'8. If

desired, a different type of} receiving india diiferent type of utilization circuitaltogether, the circuit containing receiver 12 being illustrative only.

In operation the alternating current supplied from the source 4 causes either limb of each filament alternately to rise and fall in potential about the tap point of the re- F sistance 5 and therefore with respect to the potential of the grids and plates of the respective devices. As stated above, this resistance tap is located on the center of emissivity-of the filaments and the effect of altercircuit one or more'harmonics of the filament heating currents. The transformer 7 therefore both carries a detected signal component resulting fromthe interaction of the wave components of the incoming wave from 'cator from that shown may be used, or

the circuit 1, and in the case assumed, where the heating current is of low frequency, would contain one or more audible frequency components resulting from the rectifying action of the detector 3 on the current ruin the. source 4. Assuming that the compensating device 8 has characteristics similar to the detector 3 and that the circuit is suitably balanced, the transformer 10 will carry the same audible frequency components resulting from the detector action on the wave from the source 4 as are carried by the transformer 7. However, no signal components or substantially none are present in the transformer 10 since the'signal is not impressed on the circuit of the tube 8. The transformers 7 and 10 are so related as to exert a differential action on the currentresponsive means 12, the components present in one transformer neutralizing those of the other. By suitably balancing the circuit the signal may be received in the receivers 12 and the components derived from the filament heating source may be suppressed. The circuit 11 is provided for enabling a more accurate balance to be obtained between the grid circuits of the respective tubes and to cause any'phase shift which may be introduced into the wave from the source 4 by the circuit 2 to be duplicated by the circuit 11.

Referring to Fig. 2 the compensating tube 15 is shown arranged for also amplifying the received high frequency wave before it is impressed upon the detector tube 16. The device 15 therefore serves both as a compelr sator and as a radio-frequency amplifier. The source 4 and potentiometer 5 are provided as in the previous figure and if desired the compensating resistances 9 may be supplied in the circuit of this and succeeding figures in the same manner as is indicated in Fig. 1. In operation, the radio wave received from the circuit 2 is impressed upon the grid of the compensating tube 15 and is amplified, this tube possessing an amplifying property as well as a detecting property. The condenser 14 provides a by-pass for the radio-frequency components and the high frequency transformer 17 impresses the amplified radio frequency wave upon the grid of the detector tube 16. Due to themcreased amplitude of the amplified wave, detector 16 produces a much larger detected signal frequency component than does the tube 15. The low frequency detection components of both tubes 15 and 16 are impressed through the respective transformers 17 and 18 upon the circuit containing the receiver'12. Since the components resulting from the action of these tubes upon the filament heating current are equal and are applied to the receiver 12 in phase opposition they will be balanced out. The detector 16, however, greatly predominates in producing currents of the signal frequency in the circuit of the receiver 12 and hence a resultant signal component representing the amplified signal due to the amplifying action of the tube 15, is heard in the receivem 12.

In Fig. '3 the device 20 serves as a radio frequencyamplifier, the device 21 as a radio frequency amplifier and compensator, the device 22 as a detector and the devices 23 and 24 as compensators and low frequency amplifiers. The source 25 provides alternating current for heating the filaments of the tubes 20 to 24 and also provides space current for these tubes, the current being rectified by the duplex rectifier R. This rectifier has its cathodes heated from the source 25 through the transformer 26 and the alternating current energy from thesource 25 is supplied to the anodes of the rectifier through the transformer 27. A low pass filter comprising series'inductance and shunt capacity is shown inserted in the line 28 for smoothing out the rectified current wave before it is applied to the anodes of the receiving tubes. The heating current for the filaments of the signal receiving tubes is su plied through the. transformer 29. If desired the transformers 26, 27, and 29 may be combined in the one transformer having three secondaries similar to the manner indicated in Fig. 6 to be described later.

In operation the radio frequency wave received .in the circuits 1 and 2 is amplified by the device 20 which preferably is ad justed by the aid of the grid polarizing battery 31 to have a substantially straight line characteristic, that is, the variations of its space current are substantially directly proportional to the potential variations impressed on its grid. The amplified radio frequency wave is passed through the radio frequency transformer 32 to the grid circuit of the device 21 which further amplifies the received wave. The device 21 serves also as a compensating tube for neutralizin the components of audio frequency pro need by the detector 22 from the filament heating current and therefore has some rectifying action on the received wave. The amplified radio frequency Wave from the tube 21 passes through the radio frequency condenser 33 to the radio frequency transformer 34 by which it is impressed on the grid of the detector tube 22. The radio frequency component of 4 the de tected Wave in the output of the tube 22 is shunted through the condenser 35 and the low frequency components including both the signal to be received and the noise currents from the filament heating circuit pass through the high-frequency choke coil 36 to the low frequency transformer 37. Similarly the noise currents developed in the tube, 21 due to the filament heating current are passed through the high freguency s orm'er choke 38 to the low frequency tran 39. If the transformers 37 and 39 are designed so as to ofler high impedance to the passage of radio frequency components the choke coils 36 and38 may be omitted. The transformers 37 and 39 are preferably closely coupled to each other so that the currents in the primary winding of transformer 37 are repeated into the secondary windings of both transformers 37 and 39 and the currents in the primary of transformer 39 are, likewise, re eated into the secondary windings of bot of the transformers. This effect may be attained either by winding both transformers on the same core or by closely associating the transformers in a suitable manner. As a result of the mutual relation between the transformers 37 and 39 the noise currents from the filament heating circuit are balanced out in the secondaries, tliese secondaries being so related that the currents induced in a secondary from its own primary are opposed to those induced from the primary of the other transformer. Accordingly a substantially pure signal wave is impressed on the grids of the amplifiers 23 and 24' and the amplified signal is impressed through the output transformers 41 and 42 into the circuit containing the receiver'12. However, it is to be noted that if the noise currents are not completely suppressed in the secondaries of the transformers 37 and 39, the noise currents are a plied to the tubes 23 and 24 in a different Base relation from the signal cur-- rents an in such manner that the signal components induced in the secondaries of transformers 37 and 39 are amplified by tubes 23 and 24 even though these components are unequal, while the noise currents are balanced out in the transformers 41 and 42. The tubes 23 and 24 therefore may serve both to amplify the signal and to further purify the signal from noise currents.

Fig. 4 shows a circuit arrangement similar to that of Fig. 1, the main difference being placed in the common grid-cat tube 3 has been found unsatisfactory since it disturbs the balance that is necessary if the noise from the filament heating current is to be eliminated. If the grid leak is ode cir uit,

' the grid-circuit rectification of the received waves produces a, signal voltage across the c mm n sudeak r si tan e a d h nce 1n,

the grid circuits of both tubes 3 and 8.

Since the signal would thus be impressed on both tubes alike it would be neutralized in their differential output connection in thesame way as the noise components from the filament-heating current, and no signal would then be received. If the grid leak is included in the id circuit of only the detector tube an e set is produced in the detector tube which is not duplicated in the compensator tube and hence, the noise currents from the grid circuit are impressed on the receiver. By including a grid leak in the grld circuit of eachof the tubes 3 and 8, in the manner indicated in the figure, however, it has been found possible to balance the circuit so as to receive the signal and neutralize the noise currents. The condition of balance has been found dependent upon the energy incoming from the circuit 1 and it is therefore advisable to make one or both of the elements of the grid leak circuit 43-45, for example, adjustable as indicated in the drawing. The action of the circuit of Fig. 4 is essentially the same as that described of the circuit of Fig. 1. The circuit 2 may be duplicated in the 'd circuit of the tube 8 if desired as descri ed in the case of Fig. 1.

A modified type of grid leak circuit which has provedsatisfactory in practice is illustrated in Flg. 5. In this figure a single resistance 49 replaces the two resistances 43- 44 of Fig. 4, the movable tap on this resistance being adjusted to such a point that the proper amount of resistance is included in shunt of the condenser 45 with respect to the grid circuit of the tube 3 and in shunt of the condenser 46 with respect to the grid circuit of the device 8. When this proper relation exists, the signal may be received at 12 and the noise currents balanced out as described in connection with the revious figures. In Fig. 6 an arrangement 1s shown in which the detector 50 acts as a regenerative amplifier. The space current is, in the circuit of this figure, supplied from an alternating current source through suitable rectifying devices R. The source 25, through the transformer 52, supplies heating current for the filament of the regenerative amplifier 50 and the compensator 53. The transformer 54 supplies space current enerfrom the source 25 to the duplex rectifier R from which the rectified current passes to the anode circuits of the tubes 50 and 53 through the branch 55, this branch preferably containing series inductance and shunt capacity a shown for smoothing out the rectified current wave. The filaments of the rectifier tubes may be heated throu h the transformer 56 also supplied from t 0 source 25. To assist in balancing the effects of the tubes 50 and 53 upon the receiver 12, a e st n e 67 is included in th an de ircuit and is provided with a movable tap connected with 'thepositive current lead.

The radio frequency wave received from the antenna 1 causes the normally quiescent regenerative amplifier 50 to build up oscillations of the form of the received signal ac cording to the well-known regenerative detector action. The condenser 59 furnishes a path of low impedance to the high frequency component in theoutput circuit of the detector50, and low frequency audible components, including the signal and the noise currents developed from the filamentheating current of the tube 50, pass through the low frequency transformer 60 to the receiver circuit. The audible frequency components comprising the noise currents developed in the tube 53 from the filament heating current are impressed .on the trans former 61. If the circuit is suitably balahead the audio frequency currents from the two tubes, representing noise from the filament heating circuit, are balanced out in the circuit containing the receiver 12 but the signal currents are impressed on the receiver. A phase-modifying circuit 62 is shown connected in the anode circuit of the tube 53 for assisting in balancing the circuit as a whole so that the noise currents are in exact phase opposition to each other. Such a phase-modifying circuit may be employed in the circuits of the other figures, if desired.

In Figs. and 6 the anode current for the compensatin tubes, and for the detector and other tubes utilized in reproducing the signal, is derived from a source of alternating current by rectification.- As stated above, the noise-producing components in the anode current are balanced out by virtue of the opposing or compensating action between the compensator tube and the detector or other tubes, in' a manner similar in general to that described of the filament current noises. It will be obvious in view of the circuit arrangements shown in these two figures, to provide a similar anode current.

supply in the case of each of the other figures.

The invention has been illustrated as applied to a receiving and detecting circuit. It is equally applicable to a modulating circuit, both of these being similar in that they serve to combine the impressed waves and to produce components by interaction between impressed wave components. The invention is also capable of general application and is not to be considered as limited to forms shown or described, but only by the scope of the claims.

What is claimed is:

1. In combination, an electric discharge device, circuit provisions for impressing thereon waves having instantaneous components of a plurality of frequencies, a circuit for utilizing resultant current components produced in said device as a result of interaction between the impressed components of different frequency, a second electric discharge device, said devices havin cathodes, a source of alternating current or rendering said cathodes actlve to emit electrons to furnish space discharge current through said devices, and means to balance out in the utilization circuit the variations in the space discharge, current of said devices derivedfrom said alternating current while permitting the interaction current components of the impressed waves to flow in the utilization circuit.

2. in combination, a thermionic discharge device for combining waves of different frequencies impressed thereon and =for yielding components of frequencies different from the frequencies of the impressed waves andproduce by their interaction in said device, a second discharge device, said devices having cathodes, a source of alternating current for heating said cathodes, anodes and anode circuits for said devices, and a utilization circuit associated with said anode circuits in such manner that the current components in the respective anode circuits derived from the alternating heating current are balanced out in said utilization circuit and the interaction components derived from the impressed waves are supplied to said utilization circuit.

3. A wave combinin circuit comprising an electron discharge evice for producin the combination frequencies of impressed waves, a second electron discharge device, said deviceshaving cathodes, a source of alternating current for heating said cathodes, a utilization circuit for currents of the combination frequencies, associated with said devices, and means for supplying to said utilization circuit currents of the combination frequencies and to balance out from said utilization circuit the variations in the discharge currents of said devices produced by the alternating heating current.

4. In a wave combining circuit, an electron discharge device for p'roducin the combination frequencies of impresse waves, said device having a cathode, a source of alternating current for heating said cathode, a utilization circuit for currents representing the combination frequencies of the impressed waves, and a second electron discharge device having a cathode heated by alternating current from said source, said second device bein' so arranged with respect to said first discharge device and said utilization circuit that the variations in discharge current of the second dischargedevice due to the alternating current from said source neutralize in their effect upon said utilization circuit the similar variations in the discharge current of heatin the first discharge device due t o said alter- 6. In a receiving circuit, a source of incoming waves to be detected, a thermionic discharge device detector for said waves, a receiving indicator responsive to the de tected current, said detector having a cathode, a source of alternating current for said cathode, and a second thermionic' ischarge device having a cathode heated b current from said source, said discharge evices being opposed as to their effects upon said indicator.

.7. In combination, a discharge device detector for carrier waves, said detector having an anode and an electron-emitting cathode for permitting space discharge current to flow through said device, a Source of current of varying amplitude for rendering said cathode active to emit electrons, means actuated in response to variationsin said space discharge current, a second discharge device associated with said source of current and having means for producing discharge current therethrough, and circuit arrange mentsfornopposing the effects on said means of variations in the space current of the respective devices due to said current of varying am litude and for causing said means to respon to a detected component of the 1mpressed' carrier waves.

8. In combination, a circuit for detecting a component of impressed high frequency waves comprising two electron discharge devices havin anodes and filament cathodes, anode circuits therefor, a source of alternating current for heating said cathodes, current-responsive means assoclated with the anode-circuits of said devices, and means for balancing out-the effects upon said current-responsive means of variations in space current in the respective devices due to the alternating heatinfg current, and for permitting actuation 0 high frequency waves.

9. In a'signal receiving circuit, a source of incoming waves, an electron discharge tube detector, a second electron discharge tube, cathodes for said tubes, a source of alternating current for heating said cath: odes, and a receiving indicator associated with said tubes for indicatin the detected signals, the variations in disdharge current in one ,of said tubes due to the alternating heating current belilpi opposed, as to their efiects upon, said 'cator, by the similar said current responsive means due to a detected-component of saldv variations in the discharge current in the other of said tubes;

10. A wave detecting circuit comprising two discharge tubes, one of said tubes being a detector of impressed high' frequency.

energy responsive device to balance out the effects of the alternating heating current upon said device, and to permit the actuation 0 said device in response to detected high frequency current.

11. In combination, a thermionic discharge tube detector of high frequency waves, a second thermionic discharge tube, an indicator circuit, said tubes having cathodes, a source of alternating current for heating said cathodes, anodes and anode circuits for said tubes, said anode circuits each being coupled to saidindicator circuit, said couplings being so arranged as to neutralize in said indicator circuit the production of a component derived from the heating alternating current and to permit a detected component of the high frequency waves to flow in the indicator circuit.

12. In a detecting and receiving system, an electron discharge device detector for deriving low frequency components from received high frequency waves, a second electron discharge device, each of said devices having a cathode adapted to be made-active by application of current thereto, a source of low frequency alternating current for supplying said cathodes, an indicator circult, and circuit connections for applying to said indicator the detected com nents from said first discharge device an for balancing as tosaid indicator circuitthe effects of current components in the respective dis charge devices, produced by the alternat-r ing current.

i 13. In a receiving circuit, a source of incoming high frequency waves to be detected, a thermionic-discharge device detector for said waves, a receiving indicator responsive to the detected current, said detector having a cathode, a source of alternating current for heating said cathode, and a second thermionic discharge device havin "a cathode heated by current from sai source, said second thermionic device serving to amplify the incoming high frequency waves v and to impress them upon said detector,'said devices bem'g'opposed as'to the effects u on said indicator of their respective low quen output components.

14:. n combination, a source of incomin high frequency waves to be detected, a high frequency amplifier of the thermionic-discharge type having a cathode, a source of alternating current for heating said cathode,

a thermionic discharge detector for the waves amplified by said first device, said detector having a cathode heated by alternating current from said source, a receiving indicator responsive to detected current energy, said indicator being associated with the output sides of both of said devices, said devices acting in phase opposition to each other with respect to their association with said indicator.

15. In a receiving circuit for high fre-,

quency waves, a thermionic detector for said waves, a receiving indicator responsive to detected wave energy, said detector having a cathode heated by alternating current, and a compensating discharge device for compensating for the effect on said indicator of said alternating heating current, said com ensating device also being arranged to amp ify the received high frequency Waves before their detection.

16. In a receiving system for high frequency waves, a thermionic discharge detector for said waves, said detector having a cathode adapted to be heated by alternating current, a receiving indicator responsive to the detected wave energy, a compensatin discharge device for compensating for t e effect u on said indicator of said alternating heating current, and individual grid leak elements for said devices.

17. In a receiving circuit for high frequency waves, a thermionic discharge detector for said waves, regenerative circuits for said detector, a cathode for said detector,

a source of alternating current for heating said cathode, an indicator responsive to detected wave energy and a second thermionic discharge device having a cathode heated by current from said source, for compensating for the effect on said indicator of the alternating current used to heat the cathode of said detector.

18. In a receiving circuit for high frequency waves, a thermionic discharge detector for said waves, a cathode therefor,

the efiects' upon said indicator of the heating current and rectified alternating current supplied to the respective elements of said detector. 7 19. In a receiving system for high frequency waves, an electron discharge de tector having an anode, a source of alternating current and a rectifier for supplying rectified alternating current to said anode, a receiving indicator responsive to the detected high frequenc energy, and a second electron discharge evice having an anode supplied with rectified alternating current from said source for compensating for the effect upon said indicator of fluctuations in the rectified current supplied to said detector. f

20. In a receiving circuit, a source of incoming waves to be detected, a thermionic discharge device detector havin a cathode, an indicator responsive to t e detected waves, a source of alternating current for heating said cathode, a second thermionic discharge device having a cathode heated by current from said source for balancing out the effect on said indicator of a component from said alternating heating current, and a ph'ase-modi ing circuit associated with one of said evices for aiding in the balancing out of said component.

21; In a receiving circuit, a source of incoming waves to be received, a thermionic discharge device for acting on said waves, an indicator responsive to the waves from said device, a second thermionic discharge device, cathodes for said devices, a source of alternating current for heating said cathodes, said devices being opposed as to their effects on said indicator, said incoming waves being impressed only on said first device.

22. In a receiving circuit, a thermionic discharge device detector for received high frequency waves, an indicating device responsive to detected waves, a cathode for said detector, a source of alternating current for heating said cathode, and a second thermionic discharge device havin a cathode heated b current from sai source, said second evice servin solely to neutralize the effect on said indicator, of the heating current for said detector.

In witness whereof, I hereunto subscribe my name this 14th day of October, A. D.

JOHN F. FARRINGTON. 

